Depending on the applications and the desired performance specifications, different types of memories are used.
Memories of the SRAM, or static RAM, type, thus have ultra-rapid write times, required for example for computations by a microprocessor. The major difficulty with these memories is that they are volatile and that the relatively large size of the memory point does not enable a large storage capacity to be obtained in a reasonable volume.
Memories of the DRAM, or dynamic RAM, type store electrical charges in capacitors, providing a large storage capacity. However, these memories have higher write times (several tens of nanoseconds) than those of SRAM-type memories, and are also volatile, the information retention time being of around some tens of milliseconds.
Conversely, in the case of applications requiring storage of information even when power is off, solid-state memory devices which preserve information without power are also known: these devices are called non-volatile memories. For many years, various technological solutions have thus been developed, and have led to the availability of non-volatile memories which can be written and erased electrically. The following may for example be cited:                EPROMs (“Erasable Programmable Read Only Memories”), the content of which may be written electrically, but which must be exposed to UV radiation to erase the recorded data;        EEPROMs (“Electrically Erasable Programmable ROMs”), the content of which may be written and erased electrically, but which require larger semiconductor areas for their manufacture than EPROM memories, and which are therefore more costly to manufacture.        
Since both solutions mentioned above have limits to their application, manufacturers have worked on finding an ideal non-volatile memory, which can combine the following characteristics: electrical writing and erasing, high density and low cost per bit, random access, short write and read times, satisfactory life expectancy, but also low consumption and low power voltage.
Many non-volatile memories also exist, called Flash memories, which do not have the shortcomings of the EPROM or EEPROM memories mentioned above. A Flash memory is formed from multiple memory cells which may be programmed electrically on an individual basis, where a large number of cells, called a block, sector or page, can be erased simultaneously and electrically. Flash memories combine both the benefit of EPROM memories in terms of integration density, and the benefit of EEPROM memories in terms of electrical erasure.
In addition, the durability and low electrical consumption of Flash memories make them interesting for many applications: digital cameras, mobile telephones, printers, personal assistants, laptop computers, or portable acoustic reading and recording devices, USB flash drives, etc. In addition, Flash memories have no mechanical elements, giving them quite high impact resistance. In the “all-digital” era these products have become very widely used, leading to an explosion of the Flash memory market.
Most commercially available non-volatile Flash memories use charge storage as the data encoding basis. In practice, a charge-trapping layer (generally polysilicon, or a dielectric such as SiN) is encapsulated between two dielectrics in the gate stack of a MOS transistor. The presence or absence of a charge in this medium modifies the condition of the MOS transistor, and enables the state of the memory to be encoded.
More recently, other types of rewritable non-volatile memories have appeared to reduce the voltages and programming times of Flash memories; in particular FeRAM or “Ferroelectric RAM” memories, based on polarisation switching, or MRAM or “Magnetic RAM memories, which use the direction of the residual magnetic field in the active material, may be mentioned. However, FeRAM and MRAM memories present difficulties which limit their scaling-down.
To overcome these difficulties variable-resistance memories (called RRAM or “Resistive RAM” memories) are known; these are now the subject of great attention. Memories of the resistive type may have at least two “off” or “on” states, corresponding to the transition from a resistive state (“OFF” state) to a less resistive state (“ON” state).
Three types of resistive memory may be distinguished: memories using a thermochemical mechanism, memories based on a change of valency, and memories based on electrochemical metallisation.
The latter category may be based on active materials such as ion-conductive materials, which can be referred to as CBRAM or “Conductive Bridging RAM” materials (or PMC materials, for Programmable Metallization Cell) and the operation of which is based on reversible formation and rupture of a conductive filament in a solid electrolyte, by dissolution of a soluble electrode. These memories are extremely promising due to their low programming voltages (of the order of one Volt), their short programming times (<1 μs), their low consumption and their low integration cost. In addition, these memories may be integrated in the metallisation levels of the logic of the circuit (“above IC”), enabling the density to be increased. From the architecture standpoint these memories generally require a selection device, which may, for example, be a transistor or a diode.
The operation of CBRAM-type devices is based on the formation, within a solid electrolyte, of one or more metal filaments (also called “dendrites”) between two electrodes, when appropriate respective potentials are applied to these electrodes. Formation of the filament enables a given electrical conduction to be obtained between the two electrodes. By modifying the respective potentials applied to the electrodes the distribution of the filament may be modified, and by this the electrical conduction between the two electrodes may be modified. By reversing, for example, the potential between the electrodes, the metal filament may be made to disappear or to be reduced, so as to eliminate or substantially reduce the electrical conduction due to the presence of the filament. CERAM devices may thus have a two-state operation: a state called “ON” and a state called “OFF”, and by this means may act as memory cells.
FIG. 1 represents a schematic diagram of an electronic device 1 of the CERAM type.
This device 1 is formed by a stack of the Metal/Ionic Conductor/Metal type. It comprises a solid electrolyte 2, for example made of doped chalcogenide, such as GeS, positioned between a lower electrode 3, for example made of Pt, forming an inert cathode, and an upper electrode 4 comprising a portion of ionisable metal, for example made of Ag or Cu, i.e. a portion of metal which is able easily to form metal ions (in this case, Ag+ or Cu2+ ions), and forming an anode. Device 1 represented in FIG. 1 typically forms a memory point, i.e. a unit memory cell, of a memory comprising a large number of these memory devices: in this case the electrolyte is generally integrated in a “contact point”, between the two electrodes, which are organised in line in mutually perpendicular directions.
The memory state of a CBRAM memory device results from the difference of electrical resistivity between two states: ON and OFF. In the OFF state the metal ions (for example, in this case, Ag+ ions for a soluble electrode which is in Ag) originating from the ionisable metal portion are dispersed throughout the solid electrolyte. No electrical contact is thus made between the anode and the cathode, i.e. between the ionisable metal portion and the lower electrode. The solid electrolyte forms an electrically insulating area of high resistivity between the anode and the cathode.
When a positive potential V is applied to upper soluble electrode (anode) 4, an oxidation-reduction reaction takes place at this electrode, creating mobile ions 5 (FIG. 2).
In the case of a silver electrode 4, the following reaction takes place:Ag→Ag++e−.
To accomplish this, potential V applied to soluble electrode 4 should be sufficient for the redox reaction to take place.
Ions 5 then move in electrolyte 2 under the effect of the applied electrical field. The speed of movement depends on the mobility of the ion in the electrolyte in question, which guides the choice of the soluble electrode/electrolyte pair (examples: Ag/GeS; Cu/SiO2, etc.). The ions' speeds are of the order of one nm/ns.
When they arrive at inert electrode 3 (the cathode), ions 5 are reduced through the presence of electrons supplied by the electrode, leading to the growth of a metal filament 6 according to the following reaction:Ag++e−→Ag
This filament preferentially grows in the direction of soluble electrode 4.
Memory 1 then changes to the ON state (FIG. 3) when filament 6 allows contact between the two electrodes 3 and 4, making the stack conductive. This phase is called the SET of the memory.
To change to the OFF state (RESET phase of the memory), a negative voltage V is applied to upper electrode 4, leading to the dissolution of the conductive filament. To explain this dissolution, thermal (heating) and oxidation-reduction mechanisms are generally invoked.
Many studies relate to these CBRAM memories in order to improve their reliability and their performance characteristics. Among the proposed solutions the following may in particular be cited: engineering of the electrolyte (addition of doping agents, choice of new materials, annealing, UV processing, etc.), engineering of the soluble electrode and of the inert electrode, or the addition of interface(s) between the electrodes and the electrolyte.
To reduce the dimensions of CERAM memories, an architecture based on ring-shaped conductive electrodes was proposed in U.S. Pat. No. 8,022,547. This solution enables the dimensions of the active area to be reduced, without using critical photolithography steps.
However, the known solutions mentioned above have certain difficulties.
One of the difficulties of filament memories such as CBRAMs thus relates to the high dispersion of certain electrical characteristics. In particular, high dispersions of the SET and RESET voltages are measured in the memory matrices, but also within a given device, during cycling of the cell (life expectancy measurement). This dispersion is important for the reliability of these devices, and limits their large-scale integration. These limitations are also found in memories of the OXRRAM type (oxide-based resistive memories), in which the change of resistive state is related to the formation of a filament of oxygen vacancies. One of the origins invoked to explain this dispersion relates to the difficulty in controlling the size and position of the filament, which may vary from one cycle to the next in the memory cell.
Document US2011/0120856 describes a CBRAM memory enabling the shape of the filament to be controlled; to accomplish this, the electrolyte has an asymmetrical shape, such that the contact section of the electrolyte with the soluble electrode is less than the contact section of the electrolyte with the inert electrode. The shape of the spacers surrounding the electrolyte, combined with the shape of the electrolyte, enable the active region, and therefore the shape of the conductive filament, to be clearly defined.
However, this solution also has certain drawbacks.